It is well known that the trivalent holmium ion (Ho.sup.3+) is capable of producing stimulated emission at several different wavelengths across the infrared, from 0.75 to just under 4.0 .mu.m. For the purpose of generating longer wavelengths, fluoride crystals are a preferred host for the holmium ion because the energy levels are spaced sufficiently apart within the different manifolds to mitigate against rapid multiphonon non-radiative transitions which would otherwise inhibit fluorescence at wavelengths longer than about 3 .mu.m. Thus, while the Ho transition near 2.9 .mu.m has been made to lase in many different crystals including oxides and garnets, only fluorides exhibited stimulated emission beyond 3 .mu.m. It is further known that because of the rich energy level structure of Ho, a multiplicity of wavelengths can be generated through sequential transitions between intermediate levels.
One of the most interesting Ho transitions is the one near 4 .mu.m between the .sup.5 I.sub.6 and .sup.5 I.sub.5 levels. There are very few active ion-host crystal combinations that have been successfully lased this far into the infrared, and none that have demonstrated operation levels substantially greater than a few millijoules at or near room temperature. In the case of Ho:YLF, as will be described below, stimulated emission at 3.9 .mu.m was achieved, but under conditions that severely limit prospects for further energy and power scaling to levels that are of interest. The main issue limiting laser action at 3.9 .mu.m in Ho-doped crystals, including most known fluorides, is the long fluorescence lifetime of the lower .sup.5 I.sub.6 laser level coupled with the self-terminating nature of the .sup.5 I.sub.5.fwdarw..sup.5 I.sub.6 transition. The long .sup.5 I.sub.6 lifetime--up to a few milliseconds for most fluoride materials--limits the repetition rate of the corresponding laser transition, whereas the much shorter lifetime of the upper .sup.5 I.sub.5 level--typically, no more than a few 10's of microseconds, results in an effective three-level system for the laser transition. While it is known in the art that cooling of a three level laser medium can be used to more easily achieve and sustain inversion, this approach is generally considered unattractive for practical laser systems because of added complexity and weight. It has further been recognized that an alternative way to overcome an unfavorable lifetime ratio is through use of resonant pumping, whereby the upper laser level is directly excited by a narrow band source with frequency selected or tuned to match an absorption line that is dynamically connected to the upper level of the desired transition. When the resonant pump source also has a very short pulse duration (up to about 100 nanoseconds) it is said to "gain switch" the particular transition, in much the same way Q-switching a laser oscillator produces short duration pulses.
Resonant pumping for the purpose of generating mid-infrared wavelengths from activator ions in various hosts has often been employed in the prior art. For example, in the invention disclosed in U.S. Pat. No. 5,200,966 to Esterowitcz and Stoneman, the .sup.4 I.sub.11/2 upper laser state of the erbium ion was directly pumped with a pump beam at a wavelength of about 970 nm, causing the erbium ion to produce laser emission at substantially 2.8 .mu.m, corresponding to the .sup.4 I.sub.11/2.fwdarw..sup.4 I.sub.15/2 laser transition, with high efficiency at room temperature. Because high power diode laser arrays with wavelengths in the 950-980 nm range have recently become more available, there have been several successful efforts demonstrating diode pumped, power scalable cw operation from Er-doped lasers. However, pulsed operation has been more elusive at or near 3 .mu.m, even under seemingly favorable resonant pumping conditions. In another example, U.S. Pat. No. 4,330,763 to Esterowitcz and Kruer taught use of resonant pumping from a laser source at 2.06 .mu.m to achieve inversion on the .sup.7 F.sub.3.fwdarw..sup.7 F.sub.5 line at 4.1 .mu.m from terbium-doped YLF. A large ratio of non-radiative to radiative decay rates in this gain material discriminates against broad-band pumping, but allows the use of resonant, narrow-band excitation to produce laser action. Heretofore, Holmium-doped lasers have also been made which are capable of pulsed operation in the infrared region of the spectrum upon resonant pumping by radiation from Nd:YAG lasers with output near 1 .mu.m. In particular, pulsed emission at or near 3 .mu.m from Ho-doped garnets such as YAG, GGG and YALO was described wherein co-doping with suitable activator ion such as praseodymium (Pr) was utilized to allow resonant pumping near 1 .mu.m. For example, Anton in U.S. Pat. No. 5,070,507 describes a laser system wherein a Nd-doped laser operating on a non-standard line of 1.123 .mu.m is used to pump holmium laser to produce a moderately high energy output pulse at about 3 .mu.m. Key to the invention by Anton was the incorporation of holmium ion with concentrations in excess of 15% (atomic percent) and a much lower praseodymium (Pr) concentration (on the order of 0.01%). The higher Ho concentration allowed preferential lasing on the 2.94 .mu.m line in Ho-doped garnet crystals upon pumping with the 1.12 .mu.m output of a Nd:YAG laser, whereas the Pr ion served to quench the lifetime of the lower .sup.5 I.sub.7 laser level, thereby breaking the bottleneck of the normally self-terminating .sup.5 I.sub.5.fwdarw..sup.5 I.sub.6 transition.
In the early demonstrations of the long wavelength transitions in Ho.sup.3+ -doped YLF using resonant pumping of the .sup.5 S.sub.2 manifold with short pulse green lasers, laser action on the 3.9 .mu.m line was achieved as part of a sequence with other transitions, a process known in the art as cascade lasing. Specifically, using a frequency-doubled short pulse (20 ns) Nd:glass laser operating at 535 nm to pump a 1% Ho:YLF crystal, the two-line .sup.5 S.sub.2.fwdarw..sup.5 I.sub.5, .sup.5 I.sub.5.fwdarw..sup.5 I.sub.6 (1.392.mu.m, 3.914.mu.m) and .sup.5 S.sub.2.fwdarw..sup.5 I.sub.5, .sup.5 I.sub.5.fwdarw..sup.5 I.sub.7 (1.392 .mu.m, 1.673 .mu.m) cascade transitions were successfully lased at room temperature (see L. Esterowitz, R. C. Eckardt and R. E. Allen, Appi. Phys. Lett., 35,236, (1979)). Three-step laser transitions, for example at 3.4 .mu.m, 3.9 .mu.m and 2.9 .mu.m were also reported (see R. C. Eckart, L. Esterowitz and Y. P. Lee, Procs. Int'l Conf. Lasers, pp. 380 (1981)) in Ho:YLF using the longer 1 .mu.s pulse from a pulsed dye laser tuned to 535.5 nm. These and similar results were further described in U.S. Pat. No. 4,321,559 to Esterowitz and Eckardt. A key feature in these early descriptions of resonantly pumped cascade lasing was that cascade processes, whereby one laser transition sequentially pumps a lower laser transition in the same material, could be viewed as one form of resonant self-pumping. By causing population inversion to occur sequentially, cascade laser action can therefore improve the efficiency of laser transitions between intermediate manifolds, as well as produce radiation consisting of two or more wavelengths. In the case of short pulse green laser excitation of the high lying .sup.5 S.sub.2 state, cavity optics can be selected to preferentially lase a given sequence of transitions. For example, by using one set of coated optics, the excited .sup.5 S.sub.2 state population could be directly transferred to the intermediate .sup.5 I.sub.5 level, which then serves as the upper level for a subsequent 3.9 .mu.m laser transition to the .sup.5 I.sub.6 level. A different set of cavity mirrors cause the second lasing step to occur on the 1.7 .mu.m .sup.5 I.sub.5.fwdarw..sup.5 I.sub.7 line.
Yet, although prior art describing the advantages of resonant pumping and multiwavelengths cascade lasing was related nearly two decades ago, to date no practical Ho-doped laser has been constructed with one output wavelength near either the 2.9 .mu. or 3.9 .mu.m lines, using principles taught by Esterowitcz and Eckardt. One problem with prior art systems based on resonant pumping is that they require a laser with a wavelength tuned closely to an appropriate absorption band of the laser material. For example, in the case the Ho ion, lasing at 3.9 .mu.m was previously obtained only as part of a sequence of cascade transitions, by resonantly pumping the .sup.5 I.sub.8 ground state to the .sup.5 S.sub.2, .sup.5 F.sub.4 level. To increase the pumping efficiency, the green beam had to be tuned close to the appropriate absorption peak, which in fluorides is near 535 nm. This wavelength matches up poorly with most readily available commercial lasers, which is one of the factors precluding practical application of such cascade lasers. Similarly, the methods and system disclosed by Anthon for generating 2.9 .mu.m radiation from Ho-doped garnets, while recognizing the benefits to improved efficiencies that could be obtained by increasing holmium concentrations, still required a pump laser tuned to 1.1 .mu.m, which is a difficult wavelength to obtain from a practical laser system, especially if short pulse operation is desired as well. Thus, even if pump lasers with wavelengths suitable for pumping holmium could be constructed, other conditions on the pulse duration, energy, repetition rate, and beam quality may place additional limitations on practical implementation of the infrared laser system with the output power, wavelengths and efficiency desired.